A solder-bearing lead is known in which the lead includes opposed resilient clamping fingers at one end of an elongated stem, with at least one of the fingers defining an electrical contact. The clamping fingers include opposed inner surfaces which define a gap for the reception of a rigid substrate circuit device therebetween, such that the inner surface of the electrical contact clamping finger engages a contact pad on the substrate circuit device. On an outer opposite surface of the contact clamping finger, the contact clamping finger carries a solder preform. The solder preform, upon being temporarily subjected to heat in a soldering operation, initially melts and flows over opposite sides of the contact finger onto the contact pad, and then resolidifies to bond the lead to the contact pad. Solder-bearing leads of this type are shown in the U.S. Pat. Nos. 4,019,803 to M. S. Schell, and the U.S. Pat. Nos. 4,120,558 and 4,203,648 to J. Seidler.
Solder-bearing leads as above described normally are fabricated in strip form in a progressive punch-and-die from a strip of phosphor bronze base metal which has been provided with thin tin coatings on opposite sides thereof, to facilitate the subsequent making of electrical connections to the leads. During the lead fabrication process in the progressive punch-and-die, a continuous solder wire is attached to the contact fingers of the leads and subsequently clipped between the leads to form the solder preforms on the leads. Further, during the lead fabrication process the stems of the leads are formed integrally connected to an elongated continuous support rail which subsequently is clipped from the stems after the leads have been mounted on a substrate circuit device and soldered to respective contact pads on the device.
In soldering the leads after they have been mounted on their respective substrate circuit devices, a plurality of the resultant lead-substrate circuit device assemblies initially are sprayed with a suitable flux in a fluxing chamber and then positioned in a vapor condensation mass soldering apparatus. In this apparatus, the assemblies are conveyed into a vapor condensation soldering chamber where a heated vapor from a boiling working fluid condenses on the surfaces of the assemblies and quickly heats the assemblies to cause melting of the solder preforms on the leads and soldering of the leads to their respective contact pads. Generally, a primary working fluid which is a relatively expensive high boiling, thermally stable, nonflammable fluorinated organic compound, known as Flourinert FC-70 and having the composition (C.sub.5 F.sub.11).sub.3 N, is used. In addition, to prevent loss of the expensive primary working fluid by diffusion into the atmosphere, a vapor blanket formed from a less expensive chemically inert secondary fluid, known as refrigerant FC-113 and having the composition F.sub.2 C1CCFC1.sub.2, is frequently used.
The use of spray fluxing for soldering purposes as above described is disadvantageous for a number of reasons. For example, where rosin flux is used, as is common practice, a carbonaceous deposit builds up on the surfaces of immersion heaters used to produce the condensation soldering vapors, causing undesirable overheating of the heaters and thermal degradation of the primary and secondary fluids. The thermal degradation of the primary working fluid can produce a build up of hydroflouric acid in the system, which causes undesirable etching of the metal parts of the apparatus and the lead-substrate circuit device assemblies. Similarly, the thermal degradation of the secondary fluid, in the presence of organic matter, such as the soldering flux, can produce a build up of hydrochloric acid, with similar effects. In addition, the thermal decomposition of the primary working fluid may produce a toxic gas, such as perflouroisobutylene, which becomes dissolved in the working fluid. The thermal decomposition of the secondary fluid also may produce a toxic gas, such as phosgene. Under very high acid conditions in larger condensation soldering facilities, the thermal degradation of the primary working fluid also can cause the deposition of a white crystalline material (perfluorobutylamide) on cool surfaces in the apparatus.
As a net result, the use of rosin flux in the soldering operation produces conditions which cause the soldering apparatus to require frequent maintenance, cleaning, repair and/or replacement of parts. In addition, it is desirable that recovered primary working fluid be routinely filtered to remove rosin and rosin residues before the fluid is recycled into the apparatus for reuse. Further, to avoid a build up of hydrofluoric and hydrochloric acids in the system, it is necessary to chemically neutralize these acids on a continuous basis by passing the recovered primary and secondary fluids through a chemical filter, such as soda lime, before reintroducing the fluids back into the system. Continuous care and monitoring also must be exercised and correct operating procedures observed to preclude the build up of the above-mentioned undesirable toxic gases (perfluoroisobutylene or phosgene) in the system.
In view of the foregoing, it has been proposed that the solder preforms on the solder-bearing leads be of a flux-bearing type, such as rosin core solder, to eliminate, or at least reduce, the spray fluxing of the leads, and thereby reduce the amount of solder flux which is introduced into the vapor condensation soldering system and deposited on other parts of the substrate circuit devices. It has been found, however, that because of physical limitations in the size of the solder preforms, sufficient flux cannot be incorporated into the preforms to produce satisfactory soldered connections between the leads and the substrate contact pads. In accordance with the theory of this invention, this inability to form proper soldered connections with flux-bearing solder preforms in the past is attributed to the fact that the melted solder preforms tend to flow along the lead contact fingers to the stems of the leads, then along the stems, and then in a reverse direction to the contact pads, rather than directly across the sides of the contact fingers down onto the contact pads. As a result, the fluxing capability of the flux in the solder preforms has been dissipated by the time the molten solder reaches the contact pads and the solder cannot form proper soldered connections without significant supplemental fluxing.
The affinity for the melted solder preforms to flow along the contact fingers and the stems of the leads as noted above also is disclosed in the copending patent application Ser. No. 231,568, assigned to the same assignee as the subject application, filed on even date herewith in the name of C. J. Milora, and entitled "Solder-Bearing Lead Having Solder-Confining Stop Means." In this regard, the C. J. Milora application discloses the providing of solder stops on the stems of the leads to preclude flow of the molter solder along the stems to narrow portions thereof which subsequently form soldered connections with circuit paths on a printed circuit board. It has been found, however, that solder stops located in this manner do not facilitate the use of flux-bearing solder preforms without supplemental spray fluxing prior to the soldering operation.
Accordingly, a primary purpose of this invention is to provide a new and improved solder-bearing lead in which flow of molten solder from a solder preform on a lead contact finger is controlled so that the molten solder flows directly down onto an associated contact pad on a substrate circuit device to form a satisfactory soldered connection, whereby the solder preform may be of a flux-bearing type which forms the soldered connection without any significant supplemental fluxing.